Smooth brush is not working correcly
try this:

enable dyntopo, then in the options bar above smooth shading, enable relative detail, what relative detail does is that no matter how much zoomed in or out you are, the size of your strokes will be relative only to what you set in the size options of your brush.

next zoom very close to the part of the abdomen that you want to smooth, and smooth it with the correct sized brush, in theory, that should work.

it should also be noted that the more polygons you have in there, the more gentle the smoothing will be, you can read more here:

smooth brush is destroying my model,what is happening?[SOLVED]

sources for my claim:https://www.youtube.com/watch?v=tZnUgt659oI   [at the 14:15 minute mark]

hope it works.